The present invention relates to an image forming apparatus and image forming method for suppressing deterioration of an image quality due to a variation in a rear-end-void length that is a margin area of a sheet.
A color image forming apparatus employing an electrophotographic system, such as a laser printer, a digital copy machine, a facsimile machine, or a multifunction printer, typically includes photosensitive drums for each color of cyan (C), magenta (M), yellow (Y), and black (K), an image writing section for forming an electrostatic latent image on the photosensitive drums, a development device for electrostatically adsorbing toner on the electrostatic latent image formed on the photosensitive drums through a development roller, a transfer belt for transferring the toner image developed on the photosensitive drums onto a sheet, a fusing device having a heater and a press roller for fusing the toner image transferred on the sheet, a toner box for storing the toner of each color, a conveying path for feeding or discharging the sheet, a sheet feeding cassette for storing the sheet.
For such a color image forming apparatus, measures to prevent toner from scattering are applied to improve a quality of the image formed on the sheet. For example, when the toner image is fused on the sheet, thermal expansion of the press roller caused by heating of the press roller may occur, which increases an outer diameter of the press roller to increase a sheet conveying speed by an amount of the expansion. As a result, there has arisen a problem that an image or printing matter is offset out of the sheet, and the toner scatters inside the apparatus upon the transferring on the photosensitive drums. To address this problem, an image forming apparatus having a variable image mask timing, in which a change in the sheet conveying speed due to the thermal expansion of the press roller is detected, and a rear-end mask is provided at an appropriate timing, has been proposed (refer to Japanese Patent Application Laid-Open No. H7-193693).